You're right. If you don't want to give the impression you're a national carrier, then say "ranked higher than THE national carriers" not "ranked higher than ANY national carrier." US Cellular is now considering itself a national carrier. However, even in a press release they are called a tier 2 carrier. It is defined as:

*A tier-2 or tier-3 carrier is most likely a smaller, regionally-based carrier focusing on smaller networks, who can simply buy voice and data coverage from one of the big guys (a tier-1 operator-ATT, Verizon, Sprint, T-Mobile)*

So by that definition, it's highly unlikely that US Cellular is a tier 2 national carrier. Just because they have "roaming agreements" doesn't mean it's a national carrier. So Virgin Mobile, TracFone, Boost, etc. are also national carriers because they also have agreements to use someone else's network. Bottom line, US Cellular is trying to give the impression they are something they aren't. They are trying to pretend they are one of the big boys when they aren't.